U.S. patent number 6,144,006 [Application Number 08/962,550] was granted by the patent office on 2000-11-07 for method of making and/or using copper based electrodes to spot-weld aluminum.
This patent grant is currently assigned to Ford Global Technologies, Inc.. Invention is credited to Richard Lawrence Allor, Jerald Edward Jones, Sr., Dawn Roberta White.
United States Patent |
6,144,006 |
White , et al. |
November 7, 2000 |
Method of making and/or using copper based electrodes to spot-weld
aluminum
Abstract
A method for spot-welding aluminum workpieces with copper
electrodes that comprises the steps of: (a) dissolving copper and
one or more alloying elements X to yield an alloy that increases
the liquidus of Al when dissolved in molten Al, super heating the
alloy of copper and one or more elements X that normally have
little or nor solubility in copper at room temperature, such super
heating being a temperature at which X is soluble in copper, X
being selected from the group of Mo, Ta, V, and W, elements that
form monotectic or peritectic phases with copper and aluminum
devoid of an eutectic, X being present in an amount of 4-15% by
weight of the copper; (b) rapidly cooling the alloy to room
temperature to retain such elements in solid solution; (c) either
concurrently or subsequent to step (b), forming the alloy as an
electrode shape; and (d) passing current through the electrode
shape to effect spot-welding of the aluminum workpieces when
pressed thereagainst while extending the welding life of the
electrode. Super heating and rapid cooling may be carried out by
atomizing a melt of Cu and X with a pressurized gas that directs
the spray onto a target for further cooling and eventual working or
shaping.
Inventors: |
White; Dawn Roberta (Ann Arbor,
MI), Allor; Richard Lawrence (Livonia, MI), Jones, Sr.;
Jerald Edward (Golden, CO) |
Assignee: |
Ford Global Technologies, Inc.
(Dearborn, MI)
|
Family
ID: |
24575540 |
Appl.
No.: |
08/962,550 |
Filed: |
October 31, 1997 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
642182 |
May 6, 1996 |
|
|
|
|
Current U.S.
Class: |
219/91.2; 164/46;
219/119; 427/61 |
Current CPC
Class: |
B23K
11/11 (20130101); B23K 35/0205 (20130101); B23K
35/222 (20130101); C22C 1/0425 (20130101); B23K
35/402 (20130101); B22F 2999/00 (20130101); B23K
2103/10 (20180801); B22F 2999/00 (20130101); C22C
1/0425 (20130101); B22F 3/115 (20130101) |
Current International
Class: |
B23K
11/11 (20060101); C22C 1/04 (20060101); B23K
35/02 (20060101); B23K 35/00 (20060101); B23K
35/22 (20060101); B23K 35/40 (20060101); B23K
035/04 (); B23K 011/30 () |
Field of
Search: |
;219/91.2,117.1,119
;29/527.5 ;164/46,122 ;427/59,61 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1 565 945 |
|
Apr 1970 |
|
DE |
|
0 264 626 |
|
Sep 1987 |
|
DE |
|
53-120652 |
|
Oct 1978 |
|
JP |
|
6478684 |
|
Mar 1989 |
|
JP |
|
1/113183 |
|
May 1989 |
|
JP |
|
95/11107 |
|
Apr 1995 |
|
WO |
|
Primary Examiner: Heinrich; Samuel M.
Attorney, Agent or Firm: Melotik; Lorraine S. May; Roger
L.
Parent Case Text
RELATED APPLICATIONS
This is a continuation-in-part application of U.S. Ser. No.
08/642,182 filed May 6, 1996 now abandoned entitled "Method Of
Using Copper Based Electrodes To Spot-Weld Aluminum".
Claims
What is claimed is:
1. A method of making and using copper-based electrodes to
spot-weld aluminum workpieces comprising:
(a) dissolving copper and one or more alloying elements X to yield
an alloy that increases the liquidus of Al when dissolved in molten
Al, said copper and X being super heated to an elevated temperature
at which X is soluble in copper, X being selected from the group of
tungsten and vanadium elements, said elements forming phases with
copper and aluminum devoid of an eutectic, when X is present in
said copper in an amount of 4-15% by weight of the copper;
(b) rapidly cooling said alloy to room temperature while trapping X
in solid solution within said copper;
(c) either concurrently with or subsequent to step (b), forming
said alloy as an electrode shape; and
(d) passing current through said electrode shape to effect
spot-welding of said aluminum workpieces when pressed thereagainst
while suppressing formation of low melting Al/Cu eutectics and
thereby extend the welding life of the electrode.
2. The method as in claim 1 in which dissolving and rapid cooling
is carried out by atomizing a melt of copper and X by use of a high
pressure gas to form a spray, and directing the atomized spray onto
a target for further cooling or working or shaping to form the
electrode.
3. The method as in claim 1 in which dissolving and rapid cooling
is carried out by concurrently melting powders of both copper and X
by use of a thermal spray gun or a laser beam.
4. The method as in claim 1 in which said dissolving and rapid
cooling is carried out to provide a clad layer or tip on a
previously formed solid copper electrode as the target.
5. The method as in claim 1 in which dissolving and rapid cooling
is carried out by evaporation of X to form ions, which ions migrate
to a copper electrode target, said ions implanting into the outer
surface region of said copper target to form a surface treated
copper electrode having X in solid solution in said surface region
in said weight amount.
6. The method as in claim 1 in which copper and X are dissolved to
form an alloy that coats only the tip of a preformed copper
electrode thereby constituting a thin cap at the working end of the
electrode.
7. A method of using a copper-based electrode comprised of a
superheated alloy of Cu and X rapidly cooled to retain X in
solution, X being selected from tungsten and vanadium, and X being
present in the alloy in a weight amount of 4-15%, comprising:
passing current through said electrode to effect spot-welding of
aluminum workpieces while said electrode is pressed thereagainst,
said alloy suppressing the formation of eutectic alloys between the
electrode and Al workpieces which would tend to decrease the
liquidus temperature of Al, the alloy providing decreased
solubility of the electrode in molten aluminum as a result of
increasing the liquidus temperature of molten aluminum and thereby
prolonging the welding life of said electrode.
Description
TECHNICAL FIELD
This invention relates to the technology of spot-welding metallic
pieces and more particularly to spot-welding of aluminum with
increased electrical effectiveness and electrode durability.
DISCUSSION OF THE PRIOR ART
Copper electrodes have been used for some time in the spot-welding
industry because of their excellent thermal and electrical
conductivity and because copper tends to provide the best specific
resistance for the welding electrode due to thermal gradients
established in the electrode; copper also provides the best contact
resistance for the welding electrode because it retains its
rigidity and hardness with a high degree of success.
However, copper electrodes deteriorate rapidly when used to perform
mid-frequency DC spot-welding of aluminum sheet material. This is
due mainly to a low melting point eutectic alloy that forms at the
interface between the electrode and the sheet material. The
formation of the eutectic results in rapid erosion of the copper
electrode, and presents problems associated with non-uniform and
porous nugget formation and nugget shape. (nugget being the melted
material that forms the weld joint).
It is known how to dispersion harden copper electrodes for
spot-welding zinc-galvanized steel sheet to avoid brittleness at
high temperature use (see U.S. Pat. No. 4,818,283). This dispersion
hardening is carried out by dissolving molybdenum in copper above
the copper melting point, using a superheat (such as
200-1000.degree. C.), followed by rapid cooling of about 10.sup.4
.degree. C. per second. Unfortunately, such known dispersion
hardening technique fails to recognize or perceive that a certain
class of alloying ingredients will suppress the formation of
eutectics of aluminum and copper. There is no appreciation that
molybdenum, as a representative of such class, would be of service
in extending the life of copper electrodes when used to spot-weld
aluminum.
SUMMARY OF THE INVENTION
It is an object of this invention to reform the composition of
copper electrodes to suppress or retard the formation of low
melting point eutectics with aluminum by employing alloying
ingredients that retard such formation and at the same time remain
in solid solution even at room temperature.
The invention herein meets such object by a method for making and
using copper based electrodes to spot-weld aluminum workpieces that
comprises the steps of: (a) dissolving copper and one or more
alloying elements X to yield an alloy that increases the liquidus
of Al when dissolved in molten Al, the copper and X being super
heated to an elevated temperature at which X is soluble in copper,
X being selected from the group of Mo, Ta, V, and W, elements that
form monotectic or peritectic phases with copper and aluminum
devoid of a eutectic when X is present in an amount of 4-15% by
weight of the copper; (b) rapidly cooling the superheated alloy to
room temperature to retain such elements in solid solution; (c)
either concurrently or subsequent to step (b), forming the alloy as
an electrode shape; and (d), passing current through the electrode
shape to effect spot-welding of the aluminum workpieces when
pressed thereagainst while extending the welding life of the
electrodes.
Super heating and rapid cooling may be carried out by atomizing a
melt of Cu and X with a pressurized gas that directs the spray onto
a target for further cooling and eventual working or shaping. The
super heating and rapid heating may also be carried out by
concurrently melting powers of Cu and X through use of a thermal
spray gun or a laser beam. Either of these modes may be used to
create a solid electrode comprised entirely of the alloy or used to
create a clad layer or clad tip on a previously shaped copper
electrode with X being present only in such layer or tip.
Alternatively, a solution of X in the copper may be achieved by
compacting a mixture of nano-sized particles of X and Cu under
sufficient pressure to achieve virtual solution of X in Cu even at
room temperature resulting from the intimacy of the particles at
their interfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 1A are respectively phase diagrams of an alloy of Cu
and Mo, and an alloy of Al and each Mo, diagram indicating
monotectics of the alloy;
FIG. 2 is a schematic illustration of one process for carrying out
super heating and rapid cooling of this invention utilizing
atomization of a melt of copper and X, X being an element selected
from W, V, Ta and Mo;
FIG. 3 is a highly enlarged schematic illustration of the structure
of one particle formed by the process of FIG. 2;
FIG. 4 is an elevational cross-sectional view of a typical
electrode that can be formed and shaped by the process of FIG.
2;
FIG. 5 is a schematic illustration of an alternative process for
carrying out this invention employing concurrent melting of copper
and X powders by use of a thermal spray gun;
FIG. 6 is a schematic illustration of ion implantation of X
alloying material into the surface region of a copper
electrode;
FIG. 7 is a schematic illustration of yet another alternative
process of carrying out super heating and rapid cooling of this
invention employing concurrent melting of powders of copper and X
utilizing a laser beam;
FIG. 8 is a schematic elevational view of an electrode which has
been clad by the use of the process of FIG. 7;
FIG. 9 is a sectional elevational view of an electrode that has
been clad by the process of FIG. 7 but limited to the formation of
a cap or tip for the electrode;
FIG. 10 is an aluminum-molybdenum phase diagram;
FIG. 11 is a Cu--Mo--Al phase diagram, and
FIG. 12 is a Cu--Al phase diagram.
The mechanism of spot welding electrode deterioration, that occurs
during spot welding of aluminum, differs considerably from that
observed during welding of steel or galvanized steel. A non-uniform
breakdown of the strong, adherent aluminum oxide layer at the
surface of the aluminum workpiece results in severe current
constriction during welding. This current constriction produces
elevated temperatures and local melting of the aluminum. Copper is
highly soluble in molten aluminum so that after the local melting
occurs, some of the copper electrode is dissolved by it, and a low
melting point eutectic forms. As a result, in the next welding
cycle, molten material can form at a lower temperature, and
extremely rapid dissolution of the electrode, can occur. To
understand this mechanism, imagine that an attempt is made to spot
weld ice with salt electrodes. It might work well if the ice
remained completely solid during the welding process, but if a
small amount of water formed at the surface of the ice, the salt
electrode would dissolve. This is what occurs when aluminum melts
in the presence of copper.
The effect of copper in depressing the melting point of aluminum is
easily seen from an aluminum copper phase diagram. Since other
common alloying elements in aluminum sheet metal, such as silicon
and magnesium also or eutectics, the melting point of the liquid at
the interface is actually lower than the 548.degree. C. shown. A
CuMgAl ternary eutectic with a melting point nearly 100.degree. C.
lower can also form during welding of automotive sheet aluminum
alloys strengthened with Mg.
In much of the literature, deterioration of spot welding electrodes
is associated with the pickup of aluminum on the electrode.
Numerous researchers have characterized the condition of the
electrode surface. Generally, a thin layer of aluminum bearing
eutectic is found, and in some cases molten aluminum attack of the
grain boundaries in the electrode is observed. However, it is clear
that although the electrode surface assumes a silvery appearance as
the deterioration proceeds, little aluminum is transferred to the
electrode surface. Our studies show only a small amount of Cu--Al
eutectic adheres to the surface of the electrode. The mechanism of
failure is in fact dissolution of the electrode in molten aluminum
despite the "aluminum pick-up" phrase used commonly to describe
electrode deterioration. The dissolved copper must be redeposited,
since it does not remain on the electrode. Large quantities of the
Cu--Al eutectic are deposited on the workpiece. In light of the
large amount of deposited eutectic, it is not difficult to
understand the source of the high rate of electrode wear
experienced in production. However, since the Cu--Al compound
formed is silvery in color, the deposit of this material is not
readily noticed during casual observation, whereas the very small
amount of aluminum on the electrode is easily seen because its
color differs from that of the electrode.
This invention slows the deterioration of the electrode to make it
insoluble in molten aluminum; use of V, Mo, Ta, or W in the
electrode provides a significant technical difference. The use of
such elements is not to strengthen the electrode, prevent
embrittlement, provide a cooler electrode surface, or resist attack
by zinc during joining of galvanized sheet metal.
Although molten aluminum is an excellent solvent and dissolves
copper to yield an alloy with a melting point lower than that of
pure aluminum, there are a number of metals which are not readily
soluble in molten aluminum, and which in fact elevate the liquidus
temperature of aluminum when they are dissolved in it (represented
by V, Mo, Ta and W).
FIG. 10 is an aluminum-molybdenum phase diagram illustrating how
the liquidus of Al is increased. It is readily seen that only small
amounts of molybdenum can be dissolved in molten aluminum, and that
the liquidus increases rapidly along with molybdenum content. A
similar situation prevails when molybdenum is dissolved in copper.
There is no solid solubility of Mo in Cu, and the liquidus
temperature rises quickly as Mo is dissolved in molten copper. The
ternary Cu--Mo--Al phase diagram does not show the presence of any
liquid at 600.degree. C. as seen in FIG. 11. This means that if a
small amount of molybdenum can be dissolved in the electrode, it
will be much less soluble in the presence of molten aluminum during
spot welding operations than a conventional electrode.
This possibility of suppressing the formation of the low melting
point eutectic forms the basis of this invention. Electrode
materials which are formed by alloying copper with materials which
form a monotectic with both copper and aluminum create such
suppression. There are only a small number of such metals, most of
them refractories. Some candidates are vanadium (V), tantalum (Ta),
molybdenum (Mo) and tungsten (W). There are a number of
considerations in determining alloy element composition, including
the conductivity of the alloying element and the amount required in
solution with molten aluminum to significantly affect the liquidus
temperature. Table 1 shows the conductivity of some of the relevant
materials. Molybdenum has by far the best conductivity of the
refractory metals, and a relatively small quantity of it is soluble
in aluminum. Vanadium has a lower solubility in aluminum but has a
much higher resistivity. However, since vanadium has a considerably
smaller atom than Mo, it affects the conductivity of copper less
when in solution than Mo, since it will produce lower lattice
strains.
TABLE 1 ______________________________________ Resistivity of Pure
Metals (in 10-.sup.8 Wm) Copper Aluminum Molybdenum Tantalum
Vanadium ______________________________________ at 298.degree. C.
1.712 2.709 5.47 13.4 20.1 at 627.degree. C. 6.041 10.18 21.2 40.1
58.7 ______________________________________
Producing alloys of copper and these refractory materials present
important difficulties since by definition, an alloy system which
forms a monotectic is one in which the alloying elements are
mutually insoluble. In addition, the melting point of molybdenum is
very high (2630.degree. C.), and oxides readily, so conventional
ingot metallurgy is difficult and costly. Non-equilibrium
processing methods must be used to overcome these problems.
Thus, copper electrode deterioration in the spot-welding of
aluminum can be suppressed or eliminated by use of an unique family
of alloying ingredients that, when deployed at the electrode
interface, suppresses the formation of eutectic alloys between the
copper electrode and the aluminum sheet being welded. The family of
ingredients, although not usually stable in solution at room
temperature in copper, are carefully processed so that total
solution of such ingredients in copper will continue at lower
temperatures, even to room temperature and below.
To this end, the electrode interface is fabricated with alloying
ingredients X selected from the group consisting of tungsten,
vanadium, tantalum, molybdenum, elements that suppress eutectics in
Cu and Al. This group is characterized by (a) an inability to form
an eutectic with aluminum and, when the Cu--X alloy is dissolved in
aluminum, it increases the liquidus of aluminum, and (b) does not
form an eutectic with copper and instead forms a monotectic or
peritectic reaction (see the phase diagrams of FIGS. 1 and 1A). The
alloying ingredients can be added in an amount of 4-15% by weight
of the copper to assure that a sufficient amount of the alloying
element is present in the copper to exceed the limit of solubility
of the element in aluminum when copper is dissolved in aluminum
during melting as occurs during spot welding.
As shown in FIG. 2, a process that will elevate the melting
temperature of any liquid that may begin to form at the electrode
interface during spot-welding comprises firstly atomizing a super
heated melt 10 of copper and X (X being present in amount of 4-15%
by weight of the copper). Atomization is carried out by use of a
high pressure gas 11 that comminutes the melt into particles 12 of
a size of about 10-300 microns. The superheating is preferably in
the range of 1000-1500.degree. F. above the melting temperature of
pure copper. Although X is normally not soluble in copper at room
temperature, rapid cooling at a rate of 103.degree. F./minute will
trap X in the copper matrix as well as distribute it as a very
small disperoid. The collected particles 12 may then be compacted
or forged by a machine 13 to form an electrode shape 14 as shown in
FIG. 4. This subsequent cold working will produce a wrought
copper-X alloy with X remaining trapped in solid solution in a very
finely dispersed form (0.5 microns or less). As shown in FIG. 3,
the atomized particles will be a mixture of dispersed X elements 15
within a matrix 16 of copper. The rapid cooling produces particles
having a size of about 50 micrometers, with a melting temperature
of each particle being about 1800.degree. C.
Alternatively, the spray from the atomization process may be
directed onto a substrate and immediately form a coalesced bulk
product that is close to the net shape of an electrode, thus
requiring little reworking.
Instead of atomization, separate powder supplies of copper (20) and
X (21) may be milled to nano scale particles (10-30 microns) and
blended together in a proportion to accept 4-15% by weight of X
(see process in FIG. 5). The homogeneously alloyed particles 22 are
then compacted under sufficient force and heat by device 23 to
initiate the formation of a solid that has an amorphous
microstructure with virtual solid solution of X in copper. Such
formation of the amorphous microstructure may be accompanied by
heat at a temperature of about 500.degree. F. As a result of the
process of FIG. 5, X will be in the particle boundaries of the
copper, consistent with being an extremely small disperiod of X in
copper (0.5 microns spacing or less) and thereby will prevent
copper from being preferentially dissolved during the spot-welding
operation. Compaction of such nano scale particles into a forged
electrode shape, as shown in FIG. 5, will provide certain
advantages, namely high strength and ductility combined with the
necessary dispersion of the alloying element in the copper.
As shown in FIG. 6, the extended solubility of X in copper at room
temperature may also be achieved by ion implantation. A material
30, constituting X, is eradicated to form an ion vapor 31 that
migrates to copper target 32, such as a reformed copper electrode;
the ions 31 bombard the surface 33 of such electrode to create a
surface region with X in solid solution. Cladding may also be
obtained, as shown in FIG. 7, by injecting copper and X powders
(35, 36) into a laser beam 37 where the powders will be melted and
X dissolved into the copper. When the melted particles 38, from
this laser beam intersection, are deposited on a substrate 39, such
as a previously formed copper electrode, a very high cooling rate
results and large extensions of solid solubility will occur in such
a system. This, of course, results in a surface treated electrode
that will retain very high electrical conductivity in the core of
the electrode while avoiding the added expense of fabricating the
entire solid mass of the electrode with anti-eutectic forming
material that may retard conductivity somewhat. The resulting clad
electrode, formed either by ion implantation or by concurrent
melting and spraying of feed powders, will have an appearance as
that shown in FIG. 8.
Not only is the production of copper-X alloys expensive, the
addition of the alloying element lowers electrical conductivity, a
principal characteristic of the electrode. An electrode with an
uniform CuX composition will therefore be expensive and conduct
less effectively than a conventional electrode; this is offset by
the ability of the electrode to be long-lived. Cladding, however,
produces a Cu--X layer 40 (see FIG. 8) on a conventional copper
electrode that not only will have a longer life but also reduces
the need for a costly CuX alloy.
A variation of this cladding concept would be the formations of a
compound electrode 42 (see FIG. 9)by brazing a cap 43 of a copper-X
alloy onto the previously formed solid copper electrode 44. Instead
of making the whole electrode out of the more expensive Cu--X
alloy, a thin cap of the Cu--X material is brazed onto the copper
electrode using high conductivity braze material, such as silver,
thus lowering the cost of the electrode and extending its life.
While particular embodiments of the invention have been illustrated
and described, it will be obvious to those skilled in the art that
various changes and modifications may be made without departing
from the invention, and it is intended to cover in the appended
claims all such modifications and equivalents as fall within the
true spirit and scope of this invention.
* * * * *